A stripline filter in which a stripline-type resonator is provided on a dielectric board, is used in various fields (e.g., see Patent Document 1).
FIG. 1 is an equivalent circuit diagram of an existing micro stripline filter 101 derived from Patent Document 1.
The micro stripline filter 101 is a filter in which ¼ wavelength stripline resonators of two stages are comb-line coupled to each other and each ¼ wavelength stripline resonator is externally coupled to an input/output terminal through an external coupling capacity C01. Each ¼ wavelength stripline resonator has a stepped impedance structure in which the line width is different between an open end side (118A, 118B) and a short-circuited end side (117A, 117B) and thus the impedance changes, and the electrical length is increased by making the line width on the open end side (118A, 118B) larger than the line width on the short-circuited end side (117A, 117B). By providing such a stepped impedance structure, even when the line length is shortened, a required electrical length is obtained, and the stripline filter can be reduced in size.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 7-312503
In the existing stripline filter, there is a limitation on adjustment of the electrical length by change of the line width, and it is necessary to also increase the actual line length in order to greatly increase the electrical length. Therefore, the stripline filter may need to be increased in size in order to achieve a desired low resonant frequency.
Further, for the stripline filter, there is a demand to appropriately set a spurious characteristic in a frequency band higher than the filter band, and there is a request to set a harmonic resonant frequency to a desired frequency in addition to a fundamental wave resonant frequency.